Molecular aggregated morphology of mixed monolayer composed of cyanine dye and fatty acids during compression was investigated on the basis of an atomic force microscopic observation. Optical properties of the monolayers were also evaluated from a reflected spectroscopy. 3,3'-Dioctadecyl-2,2'-oxacyanine perchior ate as dye and palmitic, arachidic, lignoceric monatonic acids as fatty acid were used for monolayer composition. Absorption behavior revealed formation of J-aggregate in the monolayer of cyanine dye and fatty acids. In particular, a specific band structure was observed for (cyanine dye/palmitic acid :50/50) mixed monolayer and also confirmed -that the structure corresponds to J-aggregate. Formation of J-aggregate in the system depended on alkyl chain length of faty acid and composition ratio. For example, mixed system of monatonic acid formed J-aggregate even the case of composition ratio of 25/75 -40/60, although that of palmitic acid formed it only the case of that of 50/50. Also ternary system of the monolayer formed J-aggregate. A required phase separated structure, which functional cyanine dye molecules were confined by fatty acid molecules at a nanometer order in the mixed monolayer cannot be constructed.